1. Field of the Invention
The present invention relates to polymeric foam articles having nanometer sized cells (nanoporous polymeric foam articles) and processes for preparing such polymeric foam articles.
2. Description of Related Art
Polymeric foam articles (or simply “polymeric foams”) are common in thermal insulation applications. Many characteristics of polymeric foams affect the thermal conductivity through the foam and, hence, the effectiveness of the foam as a thermal insulator. For instance, it is known that heat transfer through polymeric foam insulation can occur by conduction, radiation and convection (see, for example, teachings in United States patent application publication 2009/0148665). In typical polymeric foam insulation the dominant mode of heat transfer is cell gas conduction, which contributes approximately 75% of the total thermal conductivity. Hence, reducing conduction of cell gas can significantly reduce heat transfer through polymeric foams.
One characteristic affecting thermal conductivity contribution of cell gas is cell size. Cell size has little influence on gas thermal conduction when the cell size is between about one micron and one millimeter in size. Above one millimeter convection behavior tends to increase thermal conductivity. When the cell size of foam is less than about one micron the gas conductivity decreases due to what is known as the Knudsen Effect (see, for example, the relationship illustrated in FIG. 1. The curve follows the methodology in Lee, et al., “Determination of a mesopore size of aerogels from thermal conductivity measurement”, Journal of Non-Crystalline Solids, March 2002, Vol. 298, pages 287-292). The Knudsen Effect is a phenomenon that results in a decrease in thermal conductivity as fewer cell gas molecules are available within each cell to collide and transfer heat within each single cell. The Knudsen Effect becomes significant as the cell size and connectivity between cells becomes on the same order of magnitude as the mean free path of the gas filling the cells. Thermal conductivity due to cell gas reduces almost in half when the cell size reduces from one micron to 300 nanometer (nm), and reduces by almost ⅔ when the cell size reduces from one micron to below 100 nm.
Homogeneous cell sizes in this range are desirable to maximize the Knudsen Effect in view of the fact that even occasional large cells can reduce the thermal insulation effect of the small (300 nm or less, preferably 150 nm or less) cells. Therefore, all things being equal, reducing the average cell size of foam to 300 nm or less and particularly to 150 nm or less is desirable to achieve lower thermal conductivity through the foam, especially in foam having a homogeneous cell size distribution. However, it is difficult to reduce the cell size without affecting other properties of a polymeric foam article.
Porosity, the ratio of void volume to foam volume, also affects the thermal conductivity of polymeric foam. Generally, decreasing porosity results in an increase in thermal conductivity. That is because thermal conductivity through the polymer network that makes up the walls defining cells of foam is typically greater than thermal conductivity across gas within the cells.
Polymeric foam having an average cell size of 300 nm or less and a porosity of greater than 0.50 is highly desirable but difficult, and highly improbable, to achieve with known blown foam technology heretofore. Notably, blown foam technology is desirable because unlike aerogel technology, for instance, blown foam technology does not require large volumes of solvents to manufacture.
In developing a process for producing foam having a particular cell size it is useful to consider the number of effective nucleation sites. Effective nucleation sites are the number of sites in a foamable polymer composition that form voids, or cells, when the foamable polymer composition expands into foam (also known as “cell density” in, for example, a paper entitled “A Process for Making Microcellular Thermoplastic Parts” by Kumar and Suh, Polymer Engineering and Science, October 1990, Vo. 30 No. 20, pages 1323-1329). By controlling the number of effective nucleation sites and the porosity one controls the average cell size of the foam. In order to achieve a desirable thermally insulating foam it is desirable to prepare polymeric foam having at least 3×1014 effective nucleation sites per cubic centimeter of foamable polymer composition and expand that to have a porosity that is greater than 0.30 (porosity percentage greater than 30%).
It would be a desirable advancement in the art of thermally insulating polymer foam to be able to prepare polymeric foam having a thickness of at least one millimeter and having at least 3×1014 effective nucleation sites per cubic centimeter of foamable polymer composition and that has expanded to have a porosity percentage that is greater than 30%. Even more desirable would be such polymeric foam that has an average cell size of 300 nm or less, preferably 250 nm or less, and more preferably 150 nm or less. Such a foam has been developed containing nanometer-sized filler particles (nanofiller) as reported in pending patent application U.S. Ser. No. 61/264,407. However, it is still more desirable to be able to prepare such a foam without requiring such a filler; that is, in an absence of nanofiller.